Multi-channel wireless networks represent a direction that most future 4G state-of-the-art wireless communication standards evolve towards, including IEEE 802.16 Wi-MAX and 3GPP Long Term Evolution (LTE). In both Wi-MAX and LTE, Orthogonal Frequency Division Multiple Access (OFDMA) is used at the physical layer. OFDMA uses a large number of orthogonal subcarriers to maximize spectral efficiency, and assigns different subsets to different users to achieve multiple access. It is common knowledge that errors are inherently present in unreliable wireless channels. The important challenge in designing error control protocols in the MAC or physical layer is to effectively maximize achievable throughput in various transmission scenarios in wireless networks even when unpredictable and time-varying errors exist.
In the Wi-MAX physical layer, Hybrid Automatic Retransmission reQuest (HARQ) is adopted as an error control protocol by combining an ARQ and a Forward Error Correction (FEC). In addition, a performance of HARQ can be further improved by packet soft combining. The performance of HARQ, especially in the context of Wi-MAX, has been thoroughly investigated in an information-theoretic fashion. However, a built-in reliability in the HARQ has to scarify some degree of resilience to time-varying channel conditions. In addition, the HARQ does not exploit the cooperative diversity in multi-path transmissions, as it is designed for a point-to-point channel. Therefore, an improved scheme to serve as a replacement of HARQ in the Wi-MAX physical layer is needed to be proposed.
In the context of 802.11-based wireless networks with a single, shared wireless broadcast channel, a partial packet recovery algorithm has been proposed to revise the traditional ARQ. In this proposal, the erroneous portions of the packet would be re-transmitted rather than retransmitting the entire packet. However, since the feedback message has to explicitly describe the positions of error bits in the packet, this proposal would likely incur significant overhead. Further, this proposal is not designed to support cooperative transmissions in a typical multi-path transmission scenario.
There is another proposal related to a cooperative packet recovery algorithm in 802.11-based networks, which is referred to as a SOFT. This proposal works by combining confidence values across multiple faulty receptions to recover a clean packet. This proposal may able to significantly improve the data delivery rate in 802.11-based networks, in static wireless environments. However, realistic channel conditions are time varying and bursty in multi-channel wireless networks, such as Wi-MAX networks. Therefore, the performance of SOFT under such condition is unclear.
There is another proposal related to a protocol for cooperative packet recovery by performing opportunistic routing on groups of correctly received symbols in a packet, which is referred to a MIXIT. This proposal may take advantage of the broadcast nature of 802.11-based wireless networks and perform random network coding across correct symbols in different packets. This proposal provides end-to-end error recovery by employing Maximum Rank Distance (MRD) codes for push based blind redundancy transmission. However, this proposal heavily relies on opportunistic listening and routing properties in multi-hop 802.11 networks, and can not be effectively applied to multi-channel wireless networks, such as IEEE 802.16 Wi-MAX. Moreover, due to the bounded MRD code rates, it will generate large amount of overhead and is not able to provide flexibility on feedback based on-demand retransmission.